This is the first shaman spell in far-reaching line of spells that are one of the main features of the shaman class.
To explain it simply, this spell takes 50hitpoints form you, and in return gives you about 20mana (slightly more at higher levels but by then you will have better versions).
This spell casts quick but has a slight delay after it takes effect when you cant cast spells.
Why is this spell significant? Even if you are twinked with max FT, at level 24, getting 20 mana per cast of this spell is still a sigificant increase in mana.
A shaman skilled in use of this spell will rarely go Out Of Mana.
If you see yourself running low on mana, simply chain cast this spell. Keep an eye out on your hit points bar of course, but at the same time enjoy seeing your maan bar jump up after every cast of this spell.
A good tip as a shaman is to get as much HP regen as possible. This can be acomplished both with the HP regen items such as the ikksar ceremonial breastplate and with the shaman line of spells starting with "regeneration" which help you get your mana back faster.
Also when soloing, keep some bandages with you. In a recent patch, SOE has made it so that you can bind wounds while medding.
How it relates to cannibalizing ? simply canni yourself down below 50% health (exactly how low you should get depends on how brave you are;) and then sit down (which causes you get back mana and hp faster) and start binding your wounds (massively increasing the speed of your recovery).
Keeping you hp buff up is also a good method of increasing the effectiveness of this spell.
SO, in conclusion, this is the first shaman spell that helps us get our mana back much much faster than just by sitting down and watching the grass grow. With a few tips outlined above, the effectivenss of this line can be greatly increased ensuring a more prosperous shamaning career:) enjoy.
Good luck and good hunting,